I found a nice picture, and some vague info on ebay:


Marshall PB100 Powerbrake Inductive Speaker Attenuator! You want great, overdriven tone. Your tube amp sounds its best CRANKED. But you're tired of your neighbors calling the police. Or maybe you're in the studio and you don't want your amp bleeding into everyone else's mics. How can you get monster sound at lower volume levels? Simple...Put a brake on it, A Powerbrake!
FEATURES
* 100 Watt power handling capacity
* Unit will work with 8 or 16 ohm speaker loads
* Built in cooling fan for more efficient operation
* One input jack and two output speaker jacks
* Does not require any external power source or batteries
* Built like a tank (all metal construction), the best engineered attenuator on the market
Marshall Powerbrake PB100
You want great, overdriven tone. Your tube amp sounds its best CRANKED. But you're tired of your neighbors calling the police. Or maybe you're in the studio and you don't want your amp bleeding into everyone else's mics. How can you get monster sound at lower volume levels? Simple...Put a brake on it, A Powerbrake!
The Marshall Power Brake is the power attenuator that delivers great sound at low volume levels. There are other attenuators or load boxes available, but they don't provide the right type of inductive load. Your amp needs to think it's connected to a speaker. That's what the PowerBrake does. The other guys? At best, they cause your amp to sound thin and feeble. At worst, they can blow your amp's output transformer -- Ouch! Get the Marshall Power Brake. It's safer for your amp and safer for your ears.